1. Field of the Invention
The present invention relates to an image sensor, and in particular to an image sensor comprising pixels having a vertical transfer gate.
2. Discussion of the Related Art
Monolithic image sensors comprise photodiodes and transistors formed in a silicon substrate. More specifically, such image sensors comprise an array of pixels each having a photodiode coupled to a sense node via a transfer transistor. A charge accumulated by the photodiode during an integration period can be transferred to the sense node via the transfer transistor.
FIG. 1 illustrates a pixel 100 of an image sensor formed in a P-type substrate 102. A photodiode of the pixel comprises a heavily-doped P-type layer (P+) 104 formed over an N-type well 106, formed in the P-type substrate. A transfer transistor comprises a gate stack 108 formed on the surface of the P-substrate 102 on one side of the photodiode. A drain 110 is formed of a heavily-doped N-type region, and is coupled to a sense node SN. Charges accumulated by the photodiode during an integration phase can be transferred to the sense node by applying a voltage signal to the gate electrode of the transfer transistor.
The pixel 100 is insulated from adjacent pixels on each side by shallow trench isolation (STI) regions 112, 114. A spacing 116 can be provided between the STI 112 and the N-type well of the photodiode to provide electrical continuity between the surface P+ layer 104 and the P-type substrate 102, and reduce the risk of dark current generation in the photodiode.
To increase the sensitivity of the image sensor and/or reduce its size, it would be desirable to increase the surface area and depth of the N well 106. However, there are problems in increasing the size of N well 106 without limiting the charge that is transferred and/or without increasing the overall size of the image sensor.